The Russian Mosin Nagant Forum

I just purchased my first Mosin this past weekend at a gun show. I was sold an M28 which I was led to believe chambered the 7.62x54R. (It says this on the price tag, and the bill of sale for the firearm. However, the import stamp says it chambers 53R. I am unable to find a “D” stamp anywhere on the rifle. From my research I have gathered that using the 148 grain light ball ammunition is ok, so I have elected to use red army standard 7.62x54R steel cased, because they have a lead core, and come in the 148 grain. Thoughts?

Check the video above, it should help you out. Slug your barrel, for bore size ie: .308, .310, .311, .312, ..313 ect...find out your chamber dimensions go from there. Lot's of this and that out there on the 7.62x54 vs. 7.62x53 It is confusing Do not use steel case heavy ball ammo for 7.62x54 in it regardless. Steel cases do not expand and contract like brass cases do. I reload ammo for mine so, I do as required for each Mosin I shoot they can and will be different.

Video anyway is the youtube Mosin Museum (The Finnish Mosin Dilemma: What is 7.62x53r?)

53mm vs 54 mm refers to the case length. The 'D' marking indicates that the chamber was configured to be used with the D166 bullet. If you can chamber a 7.62x54R case without issue (bolt closes easily into battery and then if you chamber and extract a round unfired and find no evidence of rifling being imprinted on the bullet) then you could most likely shoot commercial 54R light ball ammunition without issue- assuming that the actual bore diameter is not at a brand new .308 diameter (meaning the barrel has some wear and maybe closer to .310 than .308).

Without a 'D' mark, it would be inadvisable to shoot D166 bullets, even if it was loaded in a 7.62x53R case. The D166 bullet is nominally a .310 diameter (7.87mm) but the length of the bullet could potentially cause issues if the chamber throat is such that the rifling is being contacted by the bullet simply by having the cartridge loaded into battery.

Only a gunsmith having inspected the rifle personally would be able to tell you what the rifle would be safe to shoot. All you will find on the internet will be maybe / probably or maybe not / probably not.